There is known a hydraulic drive system for a work machine including a regeneration circuit that reuses a hydraulic fluid discharged from a boom cylinder by an own weight fall of a boom serving as a driven body for driving an arm cylinder, in which a bottom side and a rod side of the boom cylinder are controlled in such a manner as to communicate the bottom side and the rod side with each other to boost a bottom pressure in order to increase a reuse frequency and achieve further energy saving (refer to, for example, Patent Document 1).
There is also known a hydraulic energy recovery system for recovering energy of a hydraulic fluid discharged from a boom cylinder by an own weight fall of a boom as electric energy, in which the hydraulic energy recovery system includes: a hydraulic motor that is driven by the hydraulic fluid from the boom cylinder; a power generator that is mechanically coupled to the hydraulic motor; and an electrical storage device that stores the electric energy generated by the power generator with a view to ensuring operability equivalent to that of a standard-type construction machine (work machine) without making the hydraulic energy recovery system large in size (refer to, for example, Patent Document 2). In relation to this hydraulic energy recovery system, a technique for improving regeneration efficiency by exercising control in such a manner as to communicate the bottom side and the rod side with each other to boost a bottom pressure, and for converting low pressure, high flow rate hydraulic energy into high pressure, low flow rate hydraulic energy is disclosed similarly to Patent Document 1.